Although significant advances against injury to the lung by inhaled environmental pollutants and micro-organisms will be made by research directed at specific disease entities, productive results will not always be obtained in this manner. Specific therapy may cure the acute manifestations of these diseases, but in many instances this is only after permanent damage has occurred to the lung. Moreover, man must continue to live in an environment of atmospheric pollutants, self administered pollutants, and microorganisms which will gain access to the lung. The most promising approach to the elucidation of the basis by which the body defends itself against these insults is the study of host defense mechanisms. These mechanisms include among others the mucociliary transport system. Particulate matter and micro-organisms are transported upwards to be expectorated or swallowed. Techniques for overall assessment have been developed utilizing experimental animals in our laboratory, and it is apparent that methods are now available, feasible, and safe to use in normal subjects and patients with lung diseases. Indeed, the estimation of tracheal mucous velocity by a cine bronchofiberscopic technique has now been employed by our group in over 60 studies in humans; it has permitted, for the first time, direct measurements of mucous transport over short time intervals. It has already been demonstrated that brief exposure to oxygen depresses this vital function and also that the mucous velocity is depressed in chronic obstructive pulmonary disease and cystic fibrosis. Administration of beta adrenergic agents increases mucous velocity in these conditions. However, overall velocity measurements do not differentiate whether the cilia, properties of mucus or both are affected by disease or drugs. For this reason, the frequency of ciliary beating from cells brushed from tracheobronchial and nasal mucosa, viscosity of mucus, and light and electron microscopic examination of mucosa require investigation. The mucociliary transport system has not been studied previously through an integrated approach in experimental animal models, normal subjects and patients with lung disease. An understanding of the mechanisms of the mucociliary transport will aid in ways to protect the lung from injuries due to a variety of infectious agents, inhaled pollutants, and allergic reactions.